DESCRIPTION

The wait() function suspends execution of its calling process until
status information is available for a terminated child process, or a sig-
nal is received. On return from a successful wait() call, the status
area, if non-zero, is filled in with termination information about the
process that exited (see below).
The wait4() call provides a more general interface for programs that need
to wait for certain child processes, that need resource utilization
statistics accumulated by child processes, or that require options. The
other wait functions are implemented using wait4().
The wpid parameter specifies the set of child processes for which to
wait. The following symbolic constants are currently defined in
<sys/wait.h>:
#define WAIT_ANY (-1) /* any process */
#define WAIT_MYPGRP 0 /* any process in my process group */
If wpid is set to WAIT_ANY, the call waits for any child process. If wpid
is set to WAIT_MYPGRP, the call waits for any child process in the pro-
cess group of the caller. If wpid is greater than zero, the call waits
for the process with process ID wpid. If wpid is less than -1, the call
waits for any process whose process group ID equals the absolute value of
wpid.
The status parameter is defined below. The options parameter contains the
bitwise OR of any of the following options:
WCONTINUED Causes status to be reported for stopped child processes that
have been continued by receipt of a SIGCONT signal.
WNOHANG Indicates that the call should not block if there are no
processes that wish to report status.
WUNTRACED If set, children of the current process that are stopped due
to a SIGTTIN, SIGTTOU, SIGTSTP, or SIGSTOP signal also have
their status reported.
If rusage is non-zero, a summary of the resources used by the terminated
process and all its children is returned (this information is currently
not available for stopped processes).
When the WNOHANG option is specified and no processes wish to report
status, wait4() returns a process ID of 0.
The waitpid() call is identical to wait4() with an rusage value of zero.
The older wait3() call is the same as wait4() with a wpid value of -1.
The following macros may be used to test the manner of exit of the pro-
cess. One of the first three macros will evaluate to a non-zero (true)
value:
WIFCONTINUED(status)
True if the process has not terminated, and has continued after a
job control stop. This macro can be true only if the wait call
specified the WCONTINUED option).
WIFEXITED(status)
True if the process terminated normally by a call to _exit(2) or
exit(3).
WIFSIGNALED(status)
True if the process terminated due to receipt of a signal.
WIFSTOPPED(status)
True if the process has not terminated, but has stopped and can
be restarted. This macro can be true only if the wait call speci-
fied the WUNTRACED option or if the child process is being traced
(see ptrace(2)).
Depending on the values of those macros, the following macros produce the
remaining status information about the child process:
WEXITSTATUS(status)
If WIFEXITED(status) is true, evaluates to the low-order 8 bits
of the argument passed to _exit(2) or exit(3) by the child.
WTERMSIG(status)
If WIFSIGNALED(status) is true, evaluates to the number of the
signal that caused the termination of the process.
WCOREDUMP(status)
If WIFSIGNALED(status) is true, evaluates as true if the termina-
tion of the process was accompanied by the creation of a core
file containing an image of the process when the signal was re-
ceived.
WSTOPSIG(status)
If WIFSTOPPED(status) is true, evaluates to the number of the
signal that caused the process to stop.

NOTES

See sigaction(2) for a list of termination signals. A status of 0 indi-
cates normal termination.
If a parent process terminates without waiting for all of its child
processes to terminate, the remaining child processes are assigned the
parent process 1 ID (the init process ID).
If a signal is caught while any of the wait() calls is pending, the call
may be interrupted or restarted when the signal-catching routine returns,
depending on the options in effect for the signal; for further informa-
tion, see siginterrupt(3).

RETURN VALUES

If wait() returns due to a stopped or terminated child process, the pro-
cess ID of the child is returned to the calling process. Otherwise, a
value of -1 is returned and errno is set to indicate the error.
If wait4(), wait3() or waitpid() returns due to a stopped or terminated
child process, the process ID of the child is returned to the calling
process. If there are no children not previously awaited, -1 is returned
with errno set to [ECHILD]. Otherwise, if WNOHANG is specified and there
are no stopped or exited children, 0 is returned. If an error is detected
or a caught signal aborts the call, a value of -1 is returned and errno
is set to indicate the error.

ERRORS

wait() will fail and return immediately if:
[ECHILD] The calling process has no existing unwaited-for child
processes.
[EFAULT] The status or rusage arguments point to an illegal address.
(May not be detected before exit of a child process.)
[EINTR] The call was interrupted by a caught signal, or the signal
did not have the SA_RESTART flag set.
[EINVAL] Invalid or undefined flags were passed in the options argu-
ment.

SEE ALSO

STANDARDS

The wait() and waitpid() functions are defined by POSIX; wait4() and
wait3() are not specified by POSIX. The WCOREDUMP() macro and the ability
to restart a pending wait() call are extensions to the POSIX interface.